1. MIT/WHOI Joint Program in Oceanography and Ocean Engineering and,
2. Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, USA
3. Geomicrobiology Group, Department of Biological Sciences, Marine Environmental Biology, University of Southern California, 3616 Trousdale Boulevard, Los Angeles, California 90089-0371, USA
4. Fachbereich Geowissenschaften, Universität Bremen, Postfach 33 04 40, D-28334 Bremen, Germany
5. Department of Biology, Western Washington University, Bellingham, Washington 98225, USA
6. Josephine Bay Paul Center, Marine Biological Laboratory at Woods Hole, 7 MBL Street, Woods Hole, Massachusetts 02543, USA
7. Institute of Geophysics and Planetary Physics, Scripps Institution of Oceanography, University of California, La Jolla, California 92037, USA

Correspondence to: Katrina J. Edwards^2,3 Correspondence and requests for materials should be addressed to K.J.E (Email: kje@usc.edu).

Oceanic lithosphere exposed at the sea floor undergoes seawater–rock alteration reactions involving the oxidation and hydration of glassy basalt. Basalt alteration reactions are theoretically capable of supplying sufficient energy for chemolithoautotrophic growth¹. Such reactions have been shown to generate microbial biomass in the laboratory², but field-based support for the existence of microbes that are supported by basalt alteration is lacking. Here, using quantitative polymerase chain reaction, in situ hybridization and microscopy, we demonstrate that prokaryotic cell abundances on seafloor-exposed basalts are 3–4 orders of magnitude greater than in overlying deep sea water. Phylogenetic analyses of basaltic lavas from the East Pacific Rise (9° N) and around Hawaii reveal that the basalt-hosted biosphere harbours high bacterial community richness and that community membership is shared between these sites. We hypothesize that alteration reactions fuel chemolithoautotrophic microorganisms, which constitute a trophic base of the basalt habitat, with important implications for deep-sea carbon cycling and chemical exchange between basalt and sea water.

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